To hear Alexander Zverev tell it, he would have needed to be perfect to beat Novak Djokovic in the US Open semi-finals. 

In the end, a wobbly beginning to the fifth set proved the German's undoing in a 4-6 6-2 6-4 4-6 6-2 defeat Friday simply because Djokovic was at his best when it mattered most. 

Combine that enduring quality with the Serbian's incredible statistical record, said Zverev, and you have the greatest player of all time.

Djokovic will have a chance to solidify that case Sunday when he faces Daniil Medvedev for the title. 

A victory would make the 34-year-old the first player since Rod Laver in 1969 to win all four majors in the same year and give him a record 21st grand slam title, breaking the mark he shares with Roger Federer and Rafael Nadal. 

"I think it's great for the sport," Zverev said after his loss. "Nobody thought anybody will do it again, what Rod Laver did. To see ]Djokovic] have the chance on Sunday, I do believe that he will do it is great. He's breaking every single record that there is.

"If you look at the stats, if you look the pure game of tennis action, he's the greatest of all time.

"Nobody is there with him, because most weeks world number one, most Masters 1000s titles, most likely going to be the most grand slams at the end of the day.

"And he has the chance of winning all four in the same year. How do you compete with that?"

Zverev certainly tried Friday, becoming the first player to push Djokovic to a fifth set since Stefanos Tsitsipas in the French Open final. 

"I fought back," he said. "I left it all out there. ... I mean, the match could have gone both ways, but it went his way. Very often it does."

On this night, Zverev said, it was largely Djokovic's serve on big points that boosted the top seed. 

Whatever shots happened to be working better than others for Djokovic, though, one factor stood above the rest, as it usually does for him at grand slams. 

"I think mentally he's the best player to ever play the game," Zverev said. "Mentally, in the most important moments, I would rather play against anybody else but him."